vikunja-frontend/src/styles/transitions.scss

.fade-enter-active,
.fade-leave-active {
  transition: opacity $transition-duration;
}

.fade-enter-from,
.fade-leave-to {
  opacity: 0;
}

.width-enter-active,
.width-leave-active {
  transition: width $transition-duration;
}

.width-enter-from,
.width-leave-to {
  width: 0;
}